ConcepTest 12'1a Earth and Moon I

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ConcepTest 12.1a Earth and Moon I
1) the Earth pulls harder on the Moon 2) the
Moon pulls harder on the Earth 3) they pull on
each other equally 4) there is no force between
the Earth and the Moon 5) it depends upon where
the Moon is in its orbit at that time

• Which is stronger, Earths pull on the Moon, or
  the Moons pull on Earth?

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ConcepTest 12.1a Earth and Moon I
1) the Earth pulls harder on the Moon 2) the
Moon pulls harder on the Earth 3) they pull on
each other equally 4) there is no force between
the Earth and the Moon 5) it depends upon where
the Moon is in its orbit at that time

• Which is stronger, Earths pull on the Moon, or
  the Moons pull on Earth?

By Newtons 3rd Law, the forces are equal and
opposite.
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ConcepTest 12.1b Earth and Moon II
1) one quarter 2) one half 3) the same 4)
two times 5) four times

• If the distance to the Moon were doubled, then
  the force of attraction between Earth and the
  Moon would be

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ConcepTest 12.1b Earth and Moon II
1) one quarter 2) one half 3) the same 4)
two times 5) four times

• If the distance to the Moon were doubled, then
  the force of attraction between Earth and the
  Moon would be

The gravitational force depends inversely on the
distance squared. So if you increase the
distance by a factor of 2, the force will
decrease by a factor of 4.
Follow-up What distance would increase the
force by a factor of 2?
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ConcepTest 12.5 In the Space Shuttle
1) They are so far from Earth that Earths
gravity doesnt act any more. 2) Gravitys force
pulling them inward is cancelled by the
centripetal force pushing them outward. 3) While
gravity is trying to pull them inward, they are
trying to continue on a straight-line path. 4)
Their weight is reduced in space so the force of
gravity is much weaker.

• Astronauts in the space shuttle float because

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ConcepTest 12.5 In the Space Shuttle
1) They are so far from Earth that Earths
gravity doesnt act any more. 2) Gravitys force
pulling them inward is cancelled by the
centripetal force pushing them outward. 3) While
gravity is trying to pull them inward, they are
trying to continue on a straight-line path. 4)
Their weight is reduced in space so the force of
gravity is much weaker.

• Astronauts in the space shuttle float because

Astronauts in the space shuttle float because
they are in free fall around Earth, just like a
satellite or the Moon. Again, it is gravity
that provides the centripetal force that keeps
them in circular motion.
Follow-up How weak is the value of g at an
altitude of 300 km?
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ConcepTest 12.6 Guess my Weight
If you weigh yourself at the equator of Earth,
would you get a bigger, smaller or similar value
than if you weigh yourself at one of the poles?

• 1) bigger value
• 2) smaller value
• 3) same value

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ConcepTest 12.6 Guess my Weight
If you weigh yourself at the equator of Earth,
would you get a bigger, smaller or similar value
than if you weigh yourself at one of the poles?

• 1) bigger value
• 2) smaller value
• 3) same value

The weight that a scale reads is the normal
force exerted by the floor (or the scale). At
the equator, you are in circular motion, so there
must be a net inward force toward Earths center.
This means that the normal force must be
slightly less than mg. So the scale would
register something less than your actual weight.
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ConcepTest 12.7 Force Vectors

• A planet of mass m is a distance d from Earth.
  Another planet of mass 2m is a distance 2d from
  Earth. Which force vector best represents the
  direction of the total gravitation force on Earth?

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ConcepTest 12.7 Force Vectors
A planet of mass m is a distance d from Earth.
Another planet of mass 2m is a distance 2d from
Earth. Which force vector best represents the
direction of the total gravitation force on Earth?
The force of gravity on the Earth due to m is
greater than the force due to 2m, which means
that the force component pointing down in the
figure is greater than the component pointing to
the right.
F2m GME(2m) / (2d)2 1/2 GMm / d2 Fm GME m
/ d2 GMm / d2